Low-Fire Wood David Scott Smith

Appears in the Jul/Aug 2022 issue of Pottery Making Illustrated.

When the COVID-19 pandemic hit, like most ceramics professors, I was forced to flee campus mid-semester and found myself frantically scrambling to create meaningful remote content for my students. I delivered clay to students’ houses and apartments and conducted critiques via Zoom or in backyards and fields. I met with students masked and shouting at safe distances, and interactions felt like robberies or hostage exchanges. One of my main takeaways from this experience is how woefully dependent we are on school facilities, and I was inspired to find ways that students could successfully work from home. Low-temperature wood firing in a broken, old electric kiln began as a project to help my students, but I have found the results so exciting that it has become a focus of my professional practice.

Beginning a New Journey

I experienced several lucky coincidences at the beginning of this journey. My friend Todd Hines (owner of Alligator Clay Company) gave me 1000 pounds of what he calls “Gumbo,” a clay body made by collecting, combining, then mixing all of the clay left over after cleaning the clay mixer between batches throughout a year. This mystery clay often vitrifies at low temperatures. I had also just bought Justin Rothshank’s book Low-Fire Soda, and was also familiar with Matt Kelleher’s cone-4 soda-fired earthenware. Most importantly, I had a broken electric kiln and a bunch of online students waiting for me to post something interesting. 

My first setup is still my preferred way of firing (1). I knew I wanted to spray soda into the kiln, but spraying soda directly onto the softbrick walls of an electric kiln will destroy the softbrick in seconds, so I built a firebox out of hard refractory bricks on top of a cinder-block base, and placed the electric kiln on top of it. I loaded 50 cups into that first firing, each cup glazed with either Higby Blue Raku Glaze (I didn’t have any other low-fire glazes), underglazes, or flashing slips mixed from recipes found in Rothshank’s book. I cut up a bunch of dry pallets into small pieces to use as fuel, pre-heated the kiln with my propane raku burner for a couple of hours, and then started feeding wood into the kiln.

The temperature took off like a racehorse when I removed the propane and started adding wood, and in a couple of hours, cone 1 had dropped. At the time, I was shooting for cone 4–6, but the kiln seemed to stall for a long time past 2000°F (1093°C), so eventually I sprayed soda and plugged everything up, fully expecting disappointment when the kiln cooled and I unloaded. I had videotaped portions of the firing, and was mainly happy that I had something to share online with my students. However, the next day was the best Christmas ever! The Gumbo clay was completely vitrified at cone 1, with beautiful body reduction. The slips were a range of yellows to deep red-oranges, the Higby Blue Raku glaze had beautiful copper reds on select pieces, and there was soda flashing throughout the kiln. I ended up selling each cup and made almost $2000 from that first firing. 

After the first firing, it was obvious to me that this was not a novelty project. I was amazed that I could achieve beautiful atmospheric glaze effects and body reduction at such low temperatures, without sacrificing the vibrant colors typically associated with low fire. Although the Gumbo vitrifying at a low temperature was a happy accident, I knew many earthenware bodies could be formulated to vitrify into tough, durable, utilitarian ware at cone 1 or even lower. My wife, Paula, who previously focused on electric-fired terra cotta for her pottery, became very interested in my results with low-fire wood, and all of her first experiments sold immediately. We both realized decent money could be made with only a few hours of firing, using mostly free or recycled materials (2). 

Kiln Assembly: First Approach

As I mentioned, my first design is basically an electric kiln sitting on top of a firebox of hard refractory brick, which allows me to spray soda ash and borax at the end of the firing. 

My firebox is approximately 150 standard-sized hard bricks and a couple of odd shaped bricks stacked on top of cinder-blocks (for air circulation below the kiln). Below the firebox, I have several openings that I can open/close to control air flow. The interior of the firebox is narrower than the electric kiln so that the electric kiln sits securely on top, and the corners of the firebox are used to support the first layer of kiln shelving (see 4). 

I have an old Skutt 1027 and another older kiln of indeterminate make, yet the rings are compatible (3, 4), which allows me to stack the kiln to whatever height I want. I use an old kiln shelf as the firebox door (5).

I paint the walls of the electric kiln with kiln wash (50% EPK, 50% alumina hydrate) (6), but never use kiln wash on the top cover; I don’t want the wash flaking down on my work. 

I cut an 8-inch-wide chimney hole in the center of the kiln lid (7) using a wallboard saw. After the kiln is loaded (8), I stack softbrick around the chimney hole 3–4 bricks high, and the damper is an old kiln shelf placed on top (9).

Firing the Wood Kiln

I preheat with my raku propane burner on the lowest setting (a weed burner would also work for this) (see 15). When I no longer feel humidity leaving the kiln, the temperature usually starts to climb without any adjustment needed. I start adding wood after 300°F (149°C) if the work has been bisque fired and after 500°F (260°C) if I am single-firing greenware. I start with a few small pieces while the gas burner is running—burning wood creates a quick blast of heat that can shock the ware and kiln furniture if not introduced slowly. When I shut off the gas, I’m always amazed how quickly the temperature climbs without the propane.

I use thin pieces of dry wood (10), and only add 2–3 pieces of wood at a time, unless the ash pile starts to dwindle. If that happens, then I might load up the firebox with more pieces of wood. Even if I’m using pallet slats, I will often split them with my ax. The key is to use thin, dry pieces of wood and to have the right amount of air (providing oxygen) pulled into the firebox to produce a climb in temperature.

I fire with a pyrometer. I prefer an analog pyrometer because I am a dinosaur, and watching a digital display flicker by single degrees is annoying. I add wood, watch for smoke and flame, listen for wood crackling, but mostly I watch the pyrometer for gains in temperature (11). When the kiln clears of rich flame and smoke, visibility through the peeps improves, and there is usually a small climb in temperature; I add more wood at the peak of the climb. It is better to fire slower at first and get a feel for how the temperature is rising—constantly adding wood will actually choke the firing and stall the kiln. If I see back pressure (flame/smoke) coming from peepholes, I almost never add wood, unless I’m going for heavy reduction.

I only trust the pyrometer to measure changes in temperature, but for information on heatwork, I rely on pyrometric cones positioned in the kiln so that they can be viewed through the peeps. Often the pyrometer, placed so it is reading the temperature at the top of the kiln, tells me I’m at 1700°F (927°C) when cone 2 is dropping on the bottom. I also place cones throughout the kiln to give me an idea of where the hot spots are.

Even if I am in the groove and the temperature is climbing rapidly, the kiln will often stall at certain points; when this happens I don’t freak out and start making massive adjustments to the damper or air flow. I make small adjustments (example: moving the damper out ¼ of an inch), and I allow at least 15 minutes to see how the kiln responds before I make another change. 

I like to spray soda at the end of the firing (12). I use two pounds of soda and a cup of borax diluted into one gallon of water (I boil the water, which helps dissolve the powder). I spray this solution directly into the firebox. Sometimes I shut the damper completely, or close to less than an inch, which I believe helps even distribution.

I keep a written kiln log and record time, temperature, damper adjustments, and weather at regular intervals. I can look back at successful firings and recreate the same conditions (for example, I know that having my damper open 4 inches is the sweet spot and a south wind is the best to supply oxygen to the firebox).

Kiln Assembly: Second Approach

My second design was an experiment. How could I convert an electric kiln into a wood kiln using the least amount of extra materials? In this instance I am thinking of my students, who barely have enough money for beer, much less the funds to purchase 150 firebricks. First, I carefully removed the sheet-metal cover around the lower ring of the electric kiln and removed a section of brick—this is the door to my firebox (13). I used standard-sized hard refractory brick as posts for the first level of kiln shelves. This area underneath is the firebox, and using sturdy posts ensures you don’t knock over your stack, pitching wood into the kiln. I used the same lid for this kiln as I used for the other kiln (14) (I enjoy how interchangeable the kiln sections are), and created a short chimney with softbrick and an old kiln shelf. Since I want to spray soda, and that’s not possible with this second kiln design (15), I’ve only fired this kiln once as an experiment, but it easily reached cone 1 in some areas after only 3 hours of introducing wood. It works well if you don’t have a lot of funds for materials or extra recycled materials available, or don’t want to spray soda.

Advantages to Firing Low-Fire Wood 

• All materials can be obtained for free or at a very low cost.
• One person can easily assemble the kiln and successfully fire it to temperature.
• Firings are short (4–6+ hours) and require less wood (3–4 pallets cut into 12 inch pieces will usually get me to cone 1). 
• Produces great body reduction and atmospheric slip colors. At lower temperatures, iron-rich clay bodies have a beautiful range of rich browns to black. 
• Copper reds and atmospheric glaze effects can be achieved.
• Soda effects can be achieved at low temperatures.
• Low-fire glazes, underglazes, Mason stains, and oxides remain very vibrant.
• There’s a minimum amount of cleanup after the firing and on the pots. I often reuse wadding!

Finding Materials and Supplies

A potter gave me most of my bricks for free, but I have found both bricks and used kilns very cheap or free on Craigslist or by posting my own ISO ads. Don’t be afraid to contact schools to see if they have materials they’d be willing to donate. Estate sales are great places to find kilns. If you find someone donating or selling a kiln, don’t forget to ask about kiln furniture and brick—often folks inherit kilns and don’t quite know what they are, not realizing that the kiln shelving and posts also have value.

Assembling a low-fire wood kiln with recycled materials that are free or very low cost is an excellent way to empower students. A student who cannot have an electric kiln would be able to assemble a low-fire wood kiln in a backyard, and disassemble their kiln after the firing. Students can learn how to fire with wood fuel, read a pyrometer and pyrometric cones, adjust the damper, and gauge temperature with heat color—all things that are unfortunately not covered in many ceramic programs. They can use their low-fire wood kiln to fire greenware to bisque temperatures, and they can also use the kiln to fire finished work and start their own small business. As most of us professional artists can attest, the beginning of an art career is a precarious moment, and the budding artist must find ways that they can produce work.

I love to be surprised, and low-fire wood firing has captured my imagination. I still use my electric kiln, but the recent work that I am most proud of has been fired in my low-fire wood kiln. A utilitarian potter can produce durable ware at lower temperatures, but still enjoy the benefits of firing in an atmospheric kiln. 

David Smith is associate professor of ceramics at Salisbury University (Salisbury, Maryland). He also co-owns Little Lane Pottery with his wife Paula (find us on Facebook at Little Lane Pottery). His professional work can be viewed at davidscottsmithceramics.com.